In cathode ray tube deflection systems employing an inline cathode ray tube and a deflection yoke affixed to the cathode ray tube, it is a common practice to provide horizontal and vertical deflection of the electron beams such that the electron beams sweep the whole area whereon the display is presented. However, it is known that the inline cathode ray tube in such systems tends to provide a different raster for each electron beam. Moreover, these rasters do not coincide over the total screen area and misconvergence or convergence errors result.
In an effort to compensate for this undesired non-uniformity of the resultant raster, it has been a common practice to provide convergence controls external to the cathode ray tube. These convergence controls were energized and manipulated in a manner such that the electron beams of the cathode ray tube were altered to provide a uniform raster without convergence errors.
With the appearance of and desire for a completely self-converging system wherein the desired convergence of the electron beam is automatically obtained, the prior known adjustment techniques and apparatus were no longer appropriate. Although convergence was still obtainable with such apparatus, the self-converging concept required a departure from known methods and apparatus.
Generally, in self-converging systems for inline cathode ray tubes, the same deflection force does not have an equal effect upon the central electron beam as it has on the outer electron beams. Therefore, deflection of the central electron beam is different from the deflection of the outer electron beams which causes misconvergence or the so-called coma fault. Although this coma fault is present, but hardly desirable, in small or medium size cathode ray tubes, it is an increasingly disturbing factor in cathode ray tubes known as 25V or tubes with a diagonal measurement of about 25 inches.
Additionally, present-day practice tends toward self-converging deflection systems wherein the deflection yokes include two pairs of windings on a core with one pair of windings for horizontal deflection and the other pair for vertical deflection. Moreover, these windings are commonly referred to as saddle and toroid coils or windings respectively. Also, the deflection unit is referred to as a hybrid deflection yoke with the saddle coils forming horizontal deflection windings and the toroid coils forming vertical deflection windings.
One known technique for effecting a self-converging system with an inline cathode ray tube and a hybrid type deflection yoke employs a so-called "large neck" cathode ray tube and a magnetic shunt affixed to the envelope at an appropriate place behind the deflection unit. Although, a self-converging capability without coma fault was achieved the additional neck components and space required left much to be desired. In this connection, reference is made to the article published in "FUNK-TECHNNIK" Nr. 23/1976, pages 764-767. The paragraph entitled "Coma correction in FTX deflection coils" on page 766 describes the latter solution. Moreover, other known methods for correcting the coma fault are presented in "Transactions BTR", November 1974 and entitled: "Deflection in the 20 AX system" by Kaashoek and the "20 AX System and picture tube" by Barten.
The problem in using hybrid deflection coils is also treated in an article in "IEEE Transactions On Consumer Electronics", Vol. CE-23, Nr. 3, of August 1977 by Ando, Hiroto and others entitled "New Self Convergence Yoke and Picture Tube Systems with 110.degree. In-line Feature". Therein, the coma problem is resolved, as shown in FIG. 11, page 379 for picture tubes with 110.degree. deflection by two three-dimensionally bent metal sheet parts. However, the solution is expensive due to the necessity of an elaborate, solid fixture on the last grid electrode. Moreover, experience indicates that delta tube structures having three-dimensional structures requiring a constant distance between upstanding plates are difficult to effect and it is the intention of the present invention to overcome this disadvantage.